The production of oil and natural gas from an underground well (subterranean formation) can be stimulated by a technique called hydraulic fracturing, in which a viscous fluid composition (fracturing fluid) containing a suspended proppant (e.g., sand, bauxite) is introduced into an oil or gas well via a conduit, such as tubing or casing, at a flow rate and a pressure which create, reopen and/or extend a fracture into the oil- or gas-containing formation. The proppant is carried into the fracture by the fluid composition and prevents closure of the formation after pressure is released. Leak-off of the fluid composition into the formation is limited by the fluid viscosity of the composition. Fluid viscosity also permits suspension of the proppant in the composition during the fracturing operation. Cross-linkers, such as borates, titanates or zirconates, are usually incorporated into the fluid composition to control viscosity.
Typically, less than one third of available oil is extracted from a well after it has been fractured before production rates decrease to a point at which recovery becomes uneconomical. Enhanced recovery of oil from such subterranean formations frequently involves attempting to displace the remaining crude oil with a driving fluid, e.g., gas, water, brine, steam, polymer solution, foam, or micellar solution. Ideally, such techniques (commonly called flooding techniques) provide a bank of oil of substantial depth being driven into a producing well; however, in practice this is frequently not the case. Oil-bearing strata are usually heterogeneous, some parts of them being more permeable than others. As a consequence, channeling frequently occurs, so that the driving fluid flows preferentially through permeable zones depleted of oil (so-called “thief zones”) rather than through those parts of the strata which contain sufficient oil to make oil-recovery operations profitable.
Difficulties in oil recovery due to thief zones may be corrected by injecting an aqueous solution of an organic polymer and a cross-linker into a subterranean formation under conditions where the polymer will be cross-linked to produce a gel, thus reducing permeability of the subterranean formation to the driving fluid (gas, water, etc.). Polysaccharide- or partially hydrolyzed polyacrylamide-based fluids cross-linked with certain aluminum, titanium, zirconium, and boron based compounds are used in these enhanced oil recovery applications. Cross-linked fluids or gels, whether for fracturing a subterranean formation or for reducing permeability of zones in subterranean formation, are now being used in hotter and deeper wells under a variety of temperature and pH conditions. In these operations the rate of cross-linking is critical to the successful generation of viscosity.
Boron compounds are typically used as cross-linkers in fracturing fluids used in low to mid temperature wells (150-250° F., 66-121° C.). A pH of 10 or greater is required. Cross-linking takes place immediately on mixing of boron compound with polymer base gel. Boron-cross-linked gels are not shear sensitive.
Existing delayed zirconium-based cross-linkers, based on triethanolamine or hydroxyalkylated ethylenediamine have been designed to initiate cross-linking in the wellbore. Therefore, they are ineffective at generating viscosity under mild surface temperature conditions. The gels are also shear sensitive and require higher horsepower (energy consumption) to pump.
The need exists in some fracturing fluid applications to generate an initial viscosity at the surface, followed by a delayed viscosity generation, once the fluid is subjected to higher down-hole temperatures. Current technology involves using a borate ion generating material in combination with a delayed zirconate cross-linker to accomplish both surface and delayed viscosity development. However, borate/zirconate cross-linking compositions suffer from disadvantages, such as, poor shelf stability, insufficient viscosity generation and undesirable cross-linking rates.
Most existing cross-linkers, including borate-based cross-linkers are liquid products which either freeze or become too viscous to pump under cold outdoor conditions such as found in Canada or the Rocky Mountains. There is a need for solid cross-linkers which can be used to generate high, thermally stable viscosity in a low and/or high pH environment. Such solid cross-linkers could be pre-blended with polymer, added as a solid to the aqueous polymer solution or dissolved in water and added to the aqueous polymer solution.
The need also exists for solid cross-linkers in off-shore fracturing operations, where the weight of chemicals being shipped and stored is critical. Solid cross-linkers which could contain two or more times the active metal (e.g., Zr or Ti) content of the liquid counterparts, would allow fracturing operations to be completed in a more economical fashion.
Still another need is for solid cross-linkers which are non-flammable. Many existing liquid borate, zirconate or titanate cross-linkers are flammable liquids.
U.S. Patent Application No. 2006/0058198 discloses a fluid additive comprising a cross-linker and a delay agent, wherein the cross-linker and delay agent such as sodium gluconate are combined in dry form in the field. The cross-linker may be a boron compound, a titanate, a zirconate, or a mixture thereof. In the field, the dry mixing can result in hour-to-hour or day-to-day variations due to weighing errors, incomplete mixing, or differing rates of solution.
Thus, there remain needs for solid borozirconate and borotitanate cross-linkers which are capable of generating excellent viscosity in the desired 3-5 minute range, and can be used in cold climates and off-shore applications.